A light-emitting element including a layer containing a light-emitting substance between a pair of electrodes is used as a pixel, a light source, or the like, and is provided in a light-emitting device such as a display device or a lightening system. An excited light-emitting substance emits fluorescence or phosphorescence when current is applied between a pair of electrodes of a light-emitting element.
In theory, internal quantum efficiency of phosphorescence is about three times as large as that of fluorescence in a case of current excitation, when fluorescence and phosphorescence are compared with each other. Therefore, it is considered that light-emitting efficiency is increased by using a light-emitting substance which emits phosphorescence than using a light-emitting substance which emits fluorescence, and a substance which emits phosphorescence has been developed so far.
For example, a metal complex having iridium as its central metal is disclosed in Patent Document 1. According to Patent Document 1, high-efficiency organic light-emitting device can be obtained by using the metal complex.
As described above, a light-emitting element which operates efficiently can be obtained by using a metal complex. However, metal such as iridium or platinum which is used as a central metal of a metal complex is expensive. Therefore, there is a problem that cost of raw materials for manufacturing a light-emitting element is high in a case of using a metal complex.
Patent Document 1: Published Japanese translation of PCT international application, No. 2005-506361